Multipath interference (MPI) in an optical link occurs when an optical signal can take more than one path to reach the same place. This can occur as a result of branching and recombining topologies, or as a result of reflective elements present in the link causing cavity effects. Such effects may occur as follows:
After one partial reflection in a link, a delayed version of the original signal is created, travelling in the opposite direction to the original signal. If the reflected signal is again partially reflected, a delayed version of the original signal is created which travels in the same direction as the original. It may cause interference with the original signal which can be constructive or destructive, according to the relative phase. The relative phase will depend the frequency of the signal and on the delay, which is in turn dependent on the difference in path lengths, i.e. the distance D between the reflective features.
The magnitude of the interfering signal will depend on the degree of reflection at each feature, on the gain or loss between reflections, the optical distance D, and the signal frequency. For a branching topology, path length difference, signal frequency, and path gains will characterise the MPI.
Reflections may be caused by connections, taps, optical amplifiers or isolators for example. Small amounts of reflection can cause significant interference particularly in systems containing optical amplifiers, which have gain between the reflections. This means the unwanted reflections will be amplified twice for each round-trip. Isolators are used to limit the round-trip gain, operating with a high loss in a reverse direction. However, the loss will be in the same order as the gain of the amplifier, thus the effect is only mitigated but not eliminated. MPI may vary with time as components degrade or are replaced, or as paths are switched.
Current methods for measuring MPI or parameters relating to MPI can be divided into three categories. Firstly, laboratory instruments for determining MPI effects of individual components or units will insert precise sinusoid test waveforms and include high frequency spectrum analysers for determining resultant outputs. They are not suitable for incorporation into transmission systems or for testing. They are expensive, unsuitable for field use, and incapable of operating with existing transmission sources which cannot generate pure waveforms, or be easily provided with branches to receive pure waveforms.
Secondly, methods for assessing bit error rates (BER) or signal to noise ratios (SNR) of optical transmission systems are known. They may assess the output eye, and in some circumstances, MPI may cause up to around half the noise or errors that are detected.
However, it is impossible to separate MPI from optical noise in such systems. Thus although they can perform tests under realistic operating conditions, with data traffic present, they cannot be used to derive amounts of MPI or locate sources of MPI.
Thirdly, methods of locating the cause of optical reflections are known. One example is an optical time domain reflectometer (OTDR). It is a dedicated instrument for locating reflections. It is bulky, costly, and cannot work through optical amplifiers, or while there is traffic present at the same wavelength.
Another example is known from PCT/GB95/01918 in which the function of an OTDR is incorporated in an optical element, by using the data signal as a stimulus for locating causes of reflections. The delay can be measured and thus the distance to reflective features can be calculated. This can help to locate reflective features, which is of great assistance in fault finding during commissioning.
However, such techniques can only measure reflections from points downstream of the measurement point. Furthermore, the MPI which might arise downstream of reflective features depends further on the amount of any second reflection of the reflected signal, and on any gain encountered by the twice reflected signal. These cannot be measured, and so the amount of MPI remains unknown. Furthermore, OTDR techniques cannot achieve good resolution at large distances, thus it may be difficult to distinguish closely neighbouring reflection sources.
Furthermore, if there are isolators in the path, as are usually provided in optical amplifier units, then measurements of reflections may be completely unrepresentative of MPI.
Accordingly, existing methods give no suggestion as to how to determine an amount of MPI in a link when data traffic is present. They give no suggestion as to how to determine characteristics of MPI from a measurement point downstream of sources of MPI, and no suggestion of how to derive a signature of MPI from an optical signal, or how to assess the characteristics causing the MPI.